Mean myocardial blood flow is studied with a multi-probe system, after intra-coronary-arterial injection of Xe133. Mean blood flow has been founded to be a valuable index, if measured in conjunction with another variable, such as cardiac output. Regional myocardial blood flow has been measured with intra-coronary-arterial Xe133 and a scintillation camera/computer system. Appropriate data processing program have been devised, and a mobile camera system is being constructed for use in the arteriographic suite. A method has been found to label myocardial infarcts for imaging employing bone-scanning agents of the Tc 99m-P family. The underlying principle involves mitochondrial deposition of hydroxyapatite in infarcted cells. Promising results have been obtained both in animals and human volunteers.